The present invention relates to a method for producing pump waves for Raman amplification of a signal which is passed via an optical waveguide as well as to a pump wave producing apparatus.
When an optical WDM signal (“WDM”=wavelength division multiplex) is transmitted via an optical waveguide, the attenuating characteristics of the optical waveguide lead to the signal becoming weaker. The transmitted signal therefore must be amplified after specific optical waveguide sections.
One option for signal amplification is based on the use of stimulated Raman scatter. In this case, a pump signal is fed into the optical waveguide. The pump signal is produced via a number of pump sources; for example, a number of laser diodes. In this case, the group of wavelengths from the pump sources is chosen such that, taking account of the Raman gain spectrum, all the channels in the WDM signal are amplified to the same extent (see “Fiber Optic Communication Systems”, G. P Agrawal, 2nd Edition, page 381, FIG. 8.11). By way of example, one channel is maximized at a frequency difference of 13.2 THz with respect to a pump frequency, and is amplified to a lesser extent if the frequency difference is greater or smaller. Relatively homogenous amplification of the channels in a WDM transmission signal can be achieved by using a greater number of different pump wavelengths.
Periodic filters, for example Mach-Zehnder interferometers, can be used for multiplexing the various pump wavelengths. Using Mach-Zehnder interferometers, it is possible to multiplex together pump signals with a relatively high total power, for example up to 2 W, and inject these pump signals into the optical waveguide. However, in the case of the methods which are known from the prior art using conventional Mach-Zehnder interferometers, only pump wavelength patterns with mutually equidistant pump wavelengths can be produced (see, for example, the publication “Namiki et al., Proc. OAA 2000, Quebec, OMB 2, 7-9”).
When using equidistant pump wavelengths the so-called four-wave mixing effect (“FWM”) occurs between the pump waves; particularly with fiber types with little dispersion at the pump wavelengths that are used. The four wave mixing results in the production of new frequency components, so-called mixed products, at sums or differences of pump frequencies. These are superimposed in or outside the spectrum of the pump radiation.
If the longer pump wavelengths are close to the shorter WDM signal wavelengths, the mixed products can be superimposed on the channels in the WDM signal spectrum. FIG. 1 shows an example of a signal spectrum which can occur when using 8 pump waves with equidistant pump wavelength separations. As is shown in FIG. 1, the resultant four wave mixed products FWM lie in the wavelength range of a WDM signal (in this case 1525-1605 nm).
The four wave mixing that occurs in this case results in a deterioration in the optical signal-to-noise ratio OSNR and, hence, in the signal quality on specific channels in the WDM signal. OSNR differences of approximately 8 dB can occur as a result of superimposition of mixed products in the signal spectrum.
The described effect occurs particularly severely in the case of broadband Raman amplification in C band and L band (between approximately 1525 nm and 1610 nm), since the pump wavelengths must extend virtually to the C band for amplification in L band. This results in relatively strong mixed products being produced in C band, which are superimposed on the signal spectrum. The effect is particularly pronounced in this case for fibers whose dispersion zero is close to the wavelength of the pumps with a long wavelength (approximately 1500 nm).
“Agrawal, Nonlinear Fiber Optics, 1995, p. 404” has described the physical effects which lead to four wave mixing. In principle, a distinction can be drawn between degenerate and non-degenerate four wave mixing. Non-degenerate four wave mixing is based on the interaction of photons at three different wavelengths, with photons being produced at a fourth wavelength. In contrast, in the case of degenerate four wave mixing, one wavelength is essentially included twice in the mixing process, so that degenerate four wave mixing can be produced just by photons at two different wavelengths.
It has been proposed that so-called interference filters be used instead of Mach-Zehnder interferometers for multiplexing of pump waves (see, for example, the abovementioned publication “Namaki et al., Proc. OAA 2000, Quebec, OMB 2, 7-9”). These interference filters make it possible to achieve non-equidistant intervals between the pump wavelengths. The publication “Kidorf et al., IEEE Phot. Technol. Lett., 11 (1999), 530-532” describes a non-equidistant pump wavelength distribution in which relatively more pump waves at a different wavelength are provided at shorter pump wavelengths, and relatively fewer pump waves at a different wavelength are provided at longer pump wavelengths. This makes it possible to compensate for a power transfer from short to longer wavelengths along the optical waveguide and to reduce the occurrence of four wave mixed products. However, the power which can be injected into the optical waveguide via interference filters is relatively small.
An object of the present invention is to provide a novel method for producing pump waves, and a novel pump wave producing apparatus.